1. Field of the Invention
The invention pertains to a polymer button which contains apertures which button is polished to form a contact lens which allows for the transport of dissolved oxygen along the lens/cornea interface.
2. Description of the Prior Art
The machining of polymer buttons or disks to produce contact lenses is old art and has been practiced for many decades. The buttons are generally cut from rods of solid polymer which have been prepared by extrusion or polymerization in a suitable, usually cylindrical mold or are cut or punched from sheets of polymer.
The buttons may be cylindrical and have straight vertical sides or may have a collar, depending upon the holder which is to be used to hold the button during the lathing, machining and/or polishing operation.
Hard or rigid contact lenses have traditionally been lathed from buttons containing poly(methyl methacrylate), referred to as PMMA. In addition to the PMMA homopolymer, copolymers containing a crosslinking monomer for greater rigidity and solvent resistance, as well as other comonomers have also been used in the preparation of buttons for conversion to hard contact lenses. While the inexpensive lenses comprising PMMA or its copolymers provided excellent vision correction, they caused a significant portion of the lens-wearing population to suffer from edema, i.e. redness or swelling of the eye, due to the very low oxygen permeability of the lens, and an increased corneal thickness.
While silicone rubber lenses (U.S. Pat. No. 3,228,741) were highly permeable to oxygen, they were poor optical devices and, due to their hydrophobicity, they were poorly wettable by tears and were very uncomfortable to wear. Further, they were prepared directly by casting rather than from buttons since they could not be machined or polished.
Soft hydrophilic lenses based on hydrated poly(hydroxyethyl methacrylate) (HEMA) (U.S. Pat. No. 3,220,960) have provided improved oxygen permeability by virtue of their water content. The usual procedure for the preparation of soft HEMA lenses involves either direct polymerization in a mold or rotational casting, although in some instances, the hard product produced by polymerization to a rod or button has been subjected to lathing. In either case, the shaped polymer is subjected to hydration, e.g. by soaking in saline solution, to form the soft lens. The latter is a poor optical device and is characterized by the accumulation and absorption of harmful compounds and pathological organisms, the necessity of special handling and treatment because of the high water content and difficulties in correcting astigmatic vision and other eye problems. In addition, they scratch and tear more easily than PMMA lenses. Further, a significant portion of the population still suffers from edema when wearing these soft lenses.
The invention of the rigid gas permeable (RGP) lens based on the copolymerization of polysiloxanylalkyl acrylic esters (U.S. Pat. Nos. 3,808,178; 4,120,570 and Re-issue 31,406) has overcome the edema problem. This hard, rigid copolymer is usually produced in the form of rods which are cut into buttons and lathed and polished, i.e. the same techniques used traditionally with PMMA, to form an oxygen permeable contact lens which is also an excellent optical device. However, the polysiloxanyl monomer which is responsible for the oxygen permeability is extremely expensive and a relatively high concentration is required to provide the desired gas permeability. As the concentration of the polysiloxanyl monomer is increased, the rigidity is reduced and the wettability by tears is decreased.
The prior art generally teaches either polymer compositions which act as membranes and permit gases to permeate through the contact lens or methods for providing openings in materials conventionally used in the preparation of contact lenses so as to permit the passage of tear fluid as well as the oxygen which is soluble in the aqueous fluid. The tear fluid contains epithelial cells and other metabolites which must be transported away from the cornea.
The prior art teaches the preparation of perforated or apertured contact lenses by means of devices and methods which are applied to individual contact lenses. In normal practice, the lens technician or mechanic machines or lathes a button, prepared by others who are skilled in the arts of polymerization or extrusion, to provide optical surfaces which are then polished to yield the finished contact lens. Under the prior art teachings, the mechanic would then have to use special equipment so as to subject the finished lens to the appropriate method of perforation and fenestration in order to equip the lens with apertures, followed by repolishing.
The more useful and desirable approach to an apertured contact lens involves the preparation of a button having apertures therein which is supplied to the lens maker who then proceeds in the usual manner to prepare a lathed and polished lens which has apertures by virtue of its precursor apertured button.
Japan Kokai Tokyo Koho JP 81,111,625 (Chemical Abstracts, 96, 11716 (1982) discloses the preparation of a perforated disk or button which is useful in the manufacture of contact lenses. The disks are prepared by polymerizing monomer in a plastic cylindrical container containing a number of polyvinyl chloride (PVC) fibers stretched from end to end. The polymerized product, e.g. PMMA, is removed from the cylinder and sliced to obtain disks containing pieces of PVC fibers to obtain a perforated disk.
U.S. Pat. No. 5,104,213 discloses a hard contact lens having first smaller and second larger sets of different sized apertures. Although the lenses as described in this patent did provide improvement for wearers of hard contact lenses, the lenses were expensive to make and in addition to dryness, the wearer could perceive the large center openings in the visual field.